How do you find percent composition of a compound?

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    The is the percent breakdown of the relative element masses in the molar mass (gram formula mass) of a given molecular formula.
    The percent composition of water can be found by first finding the molar mass of water.
    Water has a ratio of two atoms of hydrogen for every one atom of oxygen ##H_2O##
    hydrogen has a mass of 1.01 amu
    oxygen has a mass of 15.99 amu
    H 2 x 1.01 = 2.02
    O 1 x 15.99 = 15.99
    2.02 + 15.99 = 18.01 g/mol
    Hydrogen then has a percent composition determined by the division of 2.02 / 18.01 = 0.112 or 11.2%
    Oxygen then has a percent composition determined by the division of 15.99 / 18.01 = .887 or 88.7%
    Note that the percents only add up to 99.9% due to the fact that we rounded off the decimal values.
    To find the percent composition of a substance like aluminum hydroxide we first need to determine the formula for the molecule. In this case Aluminum is an ion of ##Al^(+3)## and the hydroxide is a polyatomic ion of ##OH^(-1)##. The formula is therefore ##Al(OH)_3##.
    aluminum has a mass of 27.9 amu
    oxygen has a mass of 15.99 amu
    hydrogen has a mass of 1.01 amu
    Al 1 x 27.9 = 27.9
    O 3 x 15.99 = 47.97 (distribute the subscript 3)
    H 3 x 1.01 = 3.03 (distribute the subscript 3)
    27.9 + 47.97 + 3.03 = 78.9 g/mol
    Therefore the percent composition breaks down as follows:
    Al 27.9 / 78.9 = .354 or 35.4%
    O 47.97 / 78.9 = .608 or 60.8%
    H 3.03 / 78.9 = .038 or 3.8%
    Here is a YouTube video for Percent Composition

    I hope this was helpful.
    SMARTERTEACHER

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